The super-secret bunker, complete with swimming pool, is rigorously guarded by security men. It is Mr. Castro's monument to his 32-year defiance of the United States and the Yankee invasion that he believes is ever-imminent.

The bunker represents Fortress Cuba, with its unyielding anti-imperialism harkening back to the 1960s when the CIA plotted to kill Mr. Castro. Its philosophy is evident everywhere, with signs that say, "Socialism or death" or "FIDELidad" (a play on fidelity).

But the warlike paranoia is wearing thin. (The artist who designed some of the signs defected last year during a Canadian stopover while en route to Moscow.) There is growing evidence that the politically astute Mr. Castro may consider "Cubastroika," or "a second phase" of his revolution as he seeks to counter the loss of East Bloc trading partners and a reduction in Soviet aid.

Among the ideas being kicked around for the upcoming Fourth Communist Party Congress are:

* A greater emphasis on private property owners in the agriculture sector, now dominated by cooperatives and collectives.

* Liberalized trade laws that would permit foreign investors for the first time to own a majority of a Cuban company.

* "Secularization" of the party to permit participation by people of religious faith.

* Privatization of small businesses, such as taxicabs, shoeshine parlors and barbershops.

* Creation of other political parties, even though it is assumed they would not be in opposition.

These ideas were broached in recent conversations with party members and young militants. One Communist militant -- a student at Havana University -- suggested there was support for creating a directly elected parliament but with the party chief (Mr. Castro) holding a veto over major decisions.

In recent elections for party posts, disaffection about maintaining the status quo was obvious. In Havana Province, many of the old guard lost elections to younger officials running anti-bureaucratic campaigns.

At Havana University -- normally a place filled with Castro thought police -- the Economics Department was allowed to debate the current economic crisis. Four math students, punished last year for suggesting that Mr. Castro should be compared to Romania's deposed dictator Nicolae Ceausescu, are back in class.

The president of the journalists' union shocked the nation in October when he confirmed his devotion to communism but said, "If there is one thing that life has shown us in a truly eloquent manner, it is that a model of a press that is officialist, apologetic and uniform has exhausted its possibilities."

His words were reported by Cuba's only daily newspaper, Granma, the Communist Party organ. The paper also took the unusual step of publishing a letter from the artists and writers union defending an exhibit that satirized Cuba's burgeoning black market system.

Granma got less generous praise in May when a disgruntled artist shocked a sculpture exhibit by publicly defecating on its latest issue. He was jailed for six months.

One of the sculptures likened the Malecon, Havana's beautiful waterfront drive, to the Berlin Wall. The artist was not punished, although his reference to the Malecon was later removed.

Such tolerated dissent is aimed at "perfecting" the system, rather than at Mr. Castro himself, who is portrayed as the father of a revolution that has given Cuba the lowest infant mortality rate and highest literacy rate in Latin America.

Those who challenge Mr. Castro directly are quickly smacked down, as was the case of the boxing fan at a televised match last year. The fan screamed, "Down with Fidel," before being pummeled into submission and hauled off by security police.

To be sure, many Cubans view the United States as the principal cause of their ills. Washington's 30-year economic blockade has cut the island nation from its former principal trading partner.

Soviet and Western European diplomats here are virtually unanimous in calling Washington's policy wrongheaded, saying it refuses to acknowledge Mr. Castro's shift away from promoting revolutions in other countries to more pacific goals.

"If the sanctions were lifted, the resulting economic influx from the U.S. would remove Castro's principal bogyman and he would be overwhelmed economically," a Western diplomat said. "Instead, the blockade has had the opposite effect, strengthening Castro's position, not weakening it. Frankly, I think it's an idiotic policy."

A State Department official said "relations would improve" if the Castro government would guarantee basic human rights, drop its military support of Salvadoran rebels and sever its military relationship with Moscow, which has several hundred military technicians here.